The present invention generally relates to charged particle beam exposure methods and apparatuses, and more particularly to a charged particle beam exposure method for making a charged particle beam exposure at a high speed and with a high accuracy and to a charged particle beam exposure apparatus which employs such a charged particle beam exposure method.
Recently, further improvements have been made with regard to the integration density and functions of integrated circuits (ICs). Hence, it is anticipated that these ICs will enhance technical progress in the industry in general, such as in computers, communication equipments and control systems for machines. The integration density of the ICs has increased by approximately four times in the recent two or three years. In the case of a dynamic random access memory (DRAM), for example, the integration density has improved considerably and the memory capacity has increased from 1 Mbit to 4 Mbit, 16 Mbit, 64 Mbit, 256 Mbit, and to even 1 Gbit. Such improvements in the integration density is due to the progresses made in forming fine patterns, and the optical technique has improved to such an extent that it is now possible to form a fine pattern having a length of 0.5 .mu.m, for example.
However, the limit of forming the fine pattern by the optical technique is about 0.4 .mu.m in length. Particularly, it is becoming extremely difficult to form a contact hole, to make a position alignment to an underlying layer and the like with an accuracy which will result in an error of 0.15 .mu.m or less.
On the other hand, charged particle beam exposures typified by an electron beam exposure are anticipated as techniques capable of overcoming the difficulties encountered in the optical technique. In addition, it is anticipated that the charged particle beam exposure can form patterns and make position alignments with an accuracy which will result in an error of less than 0.15 .mu.m at a high speed and with a high reliability.
A description will be given of a charged particle beam exposure method, by referring to FIG. 1. FIG. 1 shows a plan view of a sample which is exposed by a charged particle beam, and a plurality of chips 10 are arranged on a wafer or a glass substrate. Each chip 10 is a collection of a plurality of square cells 11 having a side of approximately 2 mm. The wafer and thus the chips 10 arranged thereon are placed on a predetermined stage (not shown) at the time of the exposure and is moved in a stage moving direction indicated by an arrow. The cells 11 which are arranged in one row along the stage moving direction form a frame 12, and each frame 12 is exposed while continuously moving the stage. Each cell 11 is formed by a square sub field 13 having a side of approximately 100 .mu.m, and a plurality of sub fields 13 arranged in a band region along a direction perpendicular to the stage moving direction as indicated by a hatching form a band 14.
The conventional charged particle beam exposure method is carried out as follows with respect to the sample described above. If the electron beam is used as the charged particle beam, the exposure is made in frames 12 in which the cells 11 are arranged along the stage moving direction. In each frame 12, each cell 11 is taken as one exposure range, and the exposure process is carried out for each band within the cell 11. In other words, by use of a main deflector which covers the entire region of the cell 11 having the side of approximately 2 mm, the electron beam is deflected in the direction perpendicular to the stage moving direction for each band 14. Hence, the electron beam is deflected to a center position of each sub field 13. For example, the main deflector is made of an electromagnetic deflector having a deflection range of .+-.1000 .mu.m. While making the above described deflection by the main deflector, a sub deflector deflects the electron beam in a fine range within the sub field 13. For example, the sub deflector is made of an electrostatic deflector having a deflection range of .+-.50 .mu.m. In addition, a shot 15 is formed by varying the electron beam size to a desired shot size by a slit deflector, and the pattern exposure is made by forming a pattern 16 which is a collection of shots 15. For example, the slit deflector is made of an electrostatic deflector having a maximum size varying range of 3 .mu.m. If the stage moves a distance equal to the width (100 .mu.m) of the band 14 during the time in which all of the patterns 16 within one band 14 can be exposed, it becomes possible to synchronize the stage movement and the exposure time and the exposure process can be carried out efficiently.
However, the density of the patterns 16 of the IC are non-uniform in general, and the exposure times of the bands 14 are not constant. If the moving speed of the stage is too fast, the stage moves past the drawing range of the main deflector and the so-called main deflector overflow occurs. In this case, some parts of the patterns 16 cannot be exposed, and the patterns cannot be exposed in a correct manner. On the other hand, if the moving speed of the stage is too slow, it takes considerable time to complete the exposure and the throughput deteriorates.
Accordingly, the exposure must be carried out at an appropriate moving speed of the stage, and the determination of the stage moving speed is an important factor.
FIG. 2 shows an example of a conventional stage controller. The stage is controlled to move in an orthogonal X-Y coordinate system, and for example, a position coordinate (X-LASER, Y-LASER) is measured by a laser interferometer or the like. The position coordinate (X-LASER, Y-LASER) is input to a digital signal processor (DSP) 20 which functions as a speed controller. The DSP 20 corrects moving speed instructions Vx and Vy for the X and Y axis with respect to set values X and Y using the position coordinate (X-LASER, Y-LASER). The corrected moving speed instruction for the X-axis is supplied to a motor 23x which drives the stage along the X-axis, via a digital-to-analog converter (DAC) 21x and an amplifier 22x. On the other hand, the corrected moving speed instruction for the Y-axis is supplied to a motor 23y which drives the stage along the Y-axis, via a DAC 21y and an amplifier 22y.
Next, a description will be given of the method of obtaining the stage moving speed in the DSP 20. In order to determine the stage moving speed, the exposure time for each band 14 is first calculated based on the number of shots 15 and the number of patterns 16 for each sub field 13. Then, the stage moving speed is set according to one of the following methods.
According to a first method, the stage moving speed is set according to a maximum exposure time out of the exposure times of each of the bands 14, and the stage is moved at the set stage moving speed which is sufficient to expose the band 14 which takes the maximum exposure time. As a result, although there is a slight waste of time during the exposure of the cell 11, the main deflection overflow is positively prevented.
According to a second method, an average stage moving speed is obtained for the bands 14 within the cell 11, and the stage moving speed is checked again for each band 14. If the exposure cannot be finished within the drawing range at the average stage moving speed, the stage moving speed is adjusted, that is, reduced in this case. In other words, the average stage moving speed is determined from the exposure times of each of the bands 14 within the cell 11, and the stage moving speed is adjusted with reference to the average stage moving speed so that all of the bands 14 can be exposed within the drawing range of .+-.1000 .mu.m, for example.
The stage moving speed may be obtained for each cell 11, and a common stage moving speed within the frame may be obtained from the stage moving speeds obtained for each of the cells 11.
However, the conventional charged particle beam exposure method suffer from the following problems when determining the stage moving speed.
First, it is necessary to know the number of bands, the number of sub fields 13, the number of patterns 16 and the number of shots 15, because the stage moving speed is calculated from these numbers. As a result, a long processing time is required to calculate the stage moving speed.
Second, an exposure current which is set when the stage moving speed is calculated may be different from an exposure current which is required to actually move the stage at the calculated stage moving speed. In this case, the stage cannot follow the change in the exposure current. In addition, the stage cannot follow a change in a parameter during the exposure.
Third, if the density of the patterns 16 greatly differs depending on the region within the sub field 13, the stage moving speed must be changed at a large number of points and the stage moving speed undergoes repeated changes. Therefore, complex calculations must be made and the processing time becomes long if the stage moving speed is to undergo gradual changes.